Is the Apple Store on Amazon a legitimate Apple retailer?

Note that the Amazon Apple store is an authorized reseller; however, independent "marketplace" resellers are not authorized resellers so be very careful whom you order from.

This is the Amazon authorized store:

https://www.amazon.com/stores/Apple/Apple/page/77D9E1F7-0337-4282-9DB6-B6B8FB2DC98D

This is not:

https://www.amazon.com/Apple-21-5-inch-3-3GHz-ME699LL-Refurbished/dp/B078ZN23LV/ref=sr_1_3?dchild=1&keywords=Apple+iMac+reseller&qid=1621108812&s=electronics&sr=1-3

(note the "ships from" and "sold by")

Yes, it is. Apple has granted the, the status of apple resellers:

https://www.lexology.com/library/detail.aspx?g=faf45bdc-f118-4641-b16f-6a5e6e8148b5